1. Field of the Invention
This invention relates to signal processing systems and, more particularly, to systems for reducing room reverberation effects.
2. Description of the Prior Art
It is well known that room reverberation can significantly reduce the perceived quality of sounds transmitted by a monaural microphone. This quality reduction is particularly disturbing in conference telephony where the nature of the room used is not generally well controlled and where, therefore, room reverberation is a factor. Other important situations where room reverberation is important include movie making, television interviews, and the like.
Room reverberations have been heuristically separated into two categories, and are defined as early echoes--perceived as spectral distortion known as "coloration"--and late reflections, or late echoes--which contribute time-domain, noise-like perceptions to speech signals.
In addition to many scholarly papers available in the art, an excellent discussion of room reverberation principles and of the methods used in the art to reduce the effect of such reverberation is presented in "Seeking the Ideal in "Hands-Free" Telephony," Berkeley et al. Bell Laboratories Record, November 1974, p. 318 et seq. Therein, the distinction between early echo distortion and late reflection distortion is discussed, together with some of the methods used for removing the different types of distortions.
In "Signal Processing to Reduce Multi-Path Distortion in Small Rooms," the Journal of the Acoustics Society of America, Vol. 7, No. 6 (Part 1) 1970, p. 1475 et seq. J. L. Flanagan describes a system for reducing early echo effects by combining the signals from two or more microphones to produce a single output signal. In accordance with the described system, the output signal of each microphone is filtered through a number of bandpass filters occupying contiguous (nonoverlapping) frequency ranges, and the microphone receiving greatest average power in a given frequency band is selected to contribute its signal to the output.
In U.S. Pat. No. 3,794,766, issued Feb. 26, 1974, Cox et al describe a system employing a multiplicity of microphones. Signal improvement is realized by equalizing the signal delay in the paths of the various microphones, and the necessary delay and equalization is determined by time-domain correlation techniques.
In copending application Ser. No. 791,418, filed Apr. 27, 1977, J. B. Allen discloses a method and apparatus for reducing both early and late echoes. The method contemplates eliminating the echoes by separating the signal of two microphones into frequency elements and by analyzing corresponding frequency elements from the microphones. Those elements which are found coherent are added and accentuated and those elements which are found not coherent are attenuated.
In another copending application, Ser. No. 791,416, filed Apr. 27, 1977, D. H. Nash discloses a system which simplifies the Allen method. Notably, Nash discloses the use of relatively few wide signal bands rather than a large plurality of narrow bands, or frequency elements, as disclosed by Allen, permitting the Nash system to conveniently operate in the time domain.